Genetic counseling: Balanced Translocation
Balanced Translocation Introduction and Contracting *Can you explain in your own words what you understand about why you were referred to genetics? *What are your main concerns for ______ at this time? *Do you have any specific questions you would like us to make sure we try to answer? Outline session *Take medical and family history *Review the chromosome results with you *Explain more about why we want to do chromosome testing on you (parents) *Dr Viskochil will examine him *We will then make sure to discuss any further questions or concerns Medical/Developmental History *follow form that I started filling out and confirm info Family History *3 generation pedigree *include standard questions don't forget miscarriages as they occur more frequently in people with balanced translocation Explain Chromosomes and Genes (using sample karyotype) *When the lab looked at _____ 's cells under a microscope they found a change in the chromosomes *The change they found was called a translocation What is a translocation? *Transfer of chromosomal material between chromosomes Why do translocations happen? *We do not really know why *Estimated that about 1 person in 500 has a chromosome translocation *We know that our chromosomes seem to break and rejoin quite often, and it is only sometimes that it leads to problems *These changes are out of our control, and nearly always translocations happen in either the egg or sperm cell before they join together *Therefore they are not likely to be caused by anything during the pregnancy How Translactions Occur *Sometimes the chromosomes break during cell division and are usually repaired *Sometimes two fragments may break off from two different chromosomes, and "swap" places *This is called a reciprocal translocation (this is the type of translocation he has) Show Karyotype and Explain Specific Break Points *Sometimes helps to see the actual karyotype *46 means that the usual number of chromosomes is present *Y chromosome is what makes him male *T stands for translocation, where a piece of chromosome 2 and 21 exchanged places *The rest just tells us more exactly where the chromosomes broke *So arrows are pointing to where the chromosomal material exchanged places Balanced Translocations *In most cases, there is no loss or gain of chromosomal material during the exchange process *When the lab looked they didn't see any missing or extra pieces of chromosomes and they therefore think that this might be a balanced translocation What Are the Effects *Often people will have a balanced translocation and not even know because they don't experience any problems (this occurs in majority of cases of new balanced rearrangements Warburton 1991) *Sometimes, apparently balanced translocation carriers show some characteristics or symptoms (Warburton showed that frequency of abnormality was 6.1% but confidence interval includes 3% in apparently balanced translocations) *One explanation for this finding is that the break may have occurred in the middle of a gene that is important in growth or development and the gene no longer provides correct instructions because it is disrupted *We do not know if this translocation is the explanation for the characteristics and health problems in ______ What will results from parental chromosomes tell us *In order to determine what the translocation might mean it is important to do chromosomes on all parents of children with a translocation *If a parent has the same translocation and they do not have any of the same symptoms or characteristics as the child than we cannot conclude that the translocation is the cause *If neither parent has the translocation than it may explain some or all of the characteristics and health problems in ______, but would be difficult to know for certain *Parental chromosomes will also be important because it will help us determine what risks might be to future offspring What can happen in offspring of people with balanced translocation *If one of the parents carries a balanced translocation, it is possible for a child of theirs to acquire a bit of extra chromosome material or a bit missing *Either of these would be known as an UNBALANCED translocation *This extra and or missing pieces of chromosome can unfortunately cause serious mental, physical and medical challenges in the child who inherits them *Sometimes they cause a miscarriage, but not always *However, it is also possible for these same parents to have a child with normal chromosomes, or a child with the same balanced rearrangement of their chromosomes as the parent *Having a balanced translocation of your chromosomes does not mean you cannot have normal healthy children *Factors that influence risks (not applicable in this case) *Ascertainment - A family ascertained as a result of a previously affected child is seen to be at increased risk of recurrence since the inbalance has been proved viable. *Risk of having an affected child is lower when a family is ascertained following for example repeated spontaneous miscarriage since in these cases the imbalance is more likely to be inviable *Each time they have a child, there are several possibilities. **they may have a child with normal chromosomes **they may have a child with the same balanced rearrangement of the chromosomes as the parent **they may have a child with an unbalanced rearrangement of the chromosomes which may cause mental and physical challenges **the pregnancy may end in a miscarriage What sort of problems do unbalanced translocations cause? *Can be difficult to predict because it depends on: *Which parts of chromosomes are missing or duplicated (aneuploidy for 4p, distal 4q and 9p for example all have a high degree of viability therefore relatively high risk) *how large of pieces they are (if large less likely to be viable due to more extra or missing information) *There are not usually the same breaking points for each translocation because it is random so we don't have any other children to compare this exact translocation to *Nearly always have learning problems though if translocation is unbalanced Tests in Pregnancy for Translocation Carriers *amniocentesis This is usually done from 14 weeks in the pregnancy and the result normally takes about 2 weeks. *Chorionic villus sampling (CVS) usually done between 10-12 weeks in the pregnancy, and the results take about 2 weeks. Psychosocial Considerations *Information can be difficult to understand because translocations are not an easy concept *Don't expect them to remember everything, we will provide a summary in a letter we will write *People who have a balanced translocation of their chromosomes do not have a disease that can be "caught" by other people *Guilt about possibly passing this on (not under our control can't cause or prevent) *What do other family members know and do you plan on telling them *Either parent considering having more children because this may alter their plans or could be upsetting to think they may have a recurrence risk if they didn't think so before References and Patient Resources *patient literature about translocations *Chromosome deletion outreach support site *UK chromosome support website (similar to chromosome deletion outreach) *information on translocations more advanced *Dorothy Warburton. De Novo Balanced Chromosome Rearrangements and extra Marker chromosomes Identified at Prenatal Diagnosis: Clinical Significance and Distribution of Breakpoints. Am J. Hum Genetics 49:995-1013, 1991. Notes The information in this outline was last updated in 2002. Material obtained under GFDL Licence from http://en.wikibooks.org/wiki/Handbook_of_Genetic_Counseling